The present invention relates generally to radio frequency (RF) signal transmitters, and more particularly to circuitry for controlling the modulation of such RF signal transmitters.
In both frequency and amplitude modulated RF signal transmitters, it is necessary to limit the degree of modulation of the transmitter due to Federal Communications Commission (FCC) requirements. Furthermore, overmodulating of the transmitter may also result in modulating signal distortion and interference with other radio communications. Conventional circuitry for limiting transmitter modulation have typically utilized back-to-back rectifiers or diodes, as shown in U.S. Pat. Nos. 2,025,595, and 2,491,590, and have also utilized limiting circuitry such as that described in U.S. Pat. Nos. 3,320,536, 3,626,331, 3,986,049 and 4,038,603, for preventing the modulating signals from exceeding a predetermined maximum peak-to-peak voltage swing. However, these circuits are non-linear in nature transforming the modulating signals into square waves. The harmonic signal content of the square waves causes the transmitted signal to have a wider than normal bandwidth. In order to keep the bandwidth of the transmitted signal within preselected limits, a so-called splatter filter is typically introduced between the modulation control circuitry and the transmitter. A splatter filter sharply attentuates signals having frequencies above 3000 Hz. However, since such splatter filters do not adequately attenuate all of the harmonic signals in the square waves, the modulating signals must be further attenuated in amplitude before application to the modulator so that the FCC modulation limit of the transmitted signal is not exceeded. Furthermore, reducing the modulating signal amplitude degrades audio signal quality since the signal-to-noise ratio is decreased.